Reproduction in Domestic Animals

Reprod Dom Anim 43 (Supp. 2), 122–128 (2008); doi: 10.1111/j.1439-0531.2008.01151.xISSN 0936-6768Genetic Aspects of Reproduction in SheepDR NotterDepartment of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, USAContentsMaintenance of high levels of realized fertility (defined as thepercentage of ewes that lamb) and appropriate levels offecundity are critical for efficient sheep production. Theoptimal level of fecundity in most situations is well below themaximum attainable level and can be targeted by combiningselection among and within breeds with use of an expandingarray of single-gene mutations affecting ovulation rate andlitter size. The heritability of litter size is approximately 0.10,allowing changes of up to 2% ⁄ year from simple mass selection.Mutations in several genes associated with the transforminggrowth factor b superfamily (BMPRIB, GDF9 and sex-linkedBMP15) can increase ovulation rates by 0.7–1.5 ova inheterozygous ewes. However, ewes that are homozygous forBMP15 or GDF9 mutations are sterile, so use of thesemutations requires carefully structured breeding programmes.Improvements in fertility may be critical for autumn lambingor programmes that aspire to lamb throughout the year.Selection to improve fertility in spring matings has beensuccessful; selected adult ewes have lambing rates of 80–85%in October and early November. The selected ewes have adramatically reduced seasonal anestrus, and many ewescontinue to cycle during spring and summer. Major genesaffecting seasonal breeding have not been identified in sheep.Polymorphisms in the melatonin receptor 1a gene appear to beassociated with seasonal breeding in some, but not all breeds.However, functional genomic studies of genes associated withcircadian and circannual rhythms have potential to revealadditional candidate genes involved in seasonal breeding.IntroductionImplementation of effective programmes of reproductivemanagement in commercial sheep productioninvolves synchronization of genetic potentials forreproductive ability with the production environment.The production objective in such situations is generallymaximization of farm profit. If meat is the main output,attainment of this objective commonly involvesmaximizing the annual number and ⁄ or total weight oflambs marketed. Wool may make an additionalcontribution to income, but unless wool quality is veryhigh, increases in wool production cannot compensatefor even minor losses in meat production. However,reproductive goals in sheep dairying, and particularlythe economic benefit from increasing litter size, may besecondary to the primary goal of increasing milkproduction.Opportunities to increase ovulation rates in sheep arevast, involving both well-established polygenic breeddifferences (Fahmy 1996) and access to a number ofsingle-gene mutations with major effects on ovulationrate and litter size. Mutant alleles may be introgressedinto different genetic backgrounds with relative ease,allowing large increases in ovulation rate without othermajor changes in genetic background or environmentaladaptation. Genetic management of ovulation rate andlitter size thus involves definition of the optimum littersize (which is commonly well below the potentiallyattainable maximum) and the development of breedingschemes to establish and maintain the desired level offecundity.Seasonal breeding limits both diversification andintensification of production. Under extensiveconditions, the breeding season of established, locallyadapted breeds generally has evolved to compliment theenvironmental conditions and desired production calendarand serves to ensure that lambing occurs at appropriatetimes. However, intensification of animalproduction to meet increased consumer demands formeat and other animal products, both locally and inexpanding global markets, provides both motivation andopportunity to modify the traditional lambing schedules.In some cases, these modifications require only a shift inthe breeding season, but there is also an opportunity toincrease the frequency of lambing, with 7- to 9-monthlambing intervals as an apparent practical minimum. Amassive literature exists cataloguing attempts to useexogenous hormone treatments or photoperiodic manipulationto induce fertile matings outside the normalbreeding season. Corresponding information now existsto show that useful levels of additive genetic variation induration of the breeding season are present among andwithin breeds, and the development of sheep with adramatically reduced breeding season will be discussed.The understanding of the genetic control of circadianand circannual timekeeping is likewise expanding rapidly,but access to quantitative trait loci (QTL) andfunctional mutations associated with seasonal breedingin sheep remains limited.This review will thus focus on genetic mechanismscontrolling ovulation rate and seasonal breeding insheep, and on strategies to synchronize geneticpotentials for reproductive traits with the productionenvironment.Genetic Control of Ovulation Rate and LitterSizePolygenic associationsA summary of heritability estimates for several reproductivetraits is shown in Table 1 (Safari et al. 2005).Estimates of heritability for litter size are generallyconsistent in the literature, averaging 0.11 (n = 102). Asdiscussed by Bradford (1985), these parameters wouldsuggest a maximum response to single-trait mass selectionfor litter size of approximately 2% per year. Thus, agenetic gain of 0.20 lambs born per ewe lambing fromsingle-trait mass selection would require approximatelyÓ 2008 The Author. Journal compilation Ó 2008 Blackwell Verlag